Assessment of Planetary Protection Requirements for Mars Sample Return Missions (2009) / Chapter Skim
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2 The Potential for Past or Present Habitable Environments on Mars
2 The Potential for Past or Present Habitable Environments on Mars
Pages 22-31
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From page 22... ...
In either case, a detailed characterization of the site prior to Mars sample return would enhance understanding of the potential for samples to contain life, whether extant or fossil, and would provide essential context for interpreting sample data, if or when samples are collected, returned, and analyzed from that site. When considering the advances in understanding of Mars that have occurred in the past decade, it is important not to forget that the scientific environment for martian studies has undergone considerable change.
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From page 23... ...
The Gamma Ray Spectrometer investigation onboard the Mars Odyssey orbiter confirmed high concentrations of water ice buried just a few centimeters below the surface in both hemispheres poleward of ~60° latitude. 2,3 The MARSIS radar experiment on Mars Express has shown that in many places this buried, ice-rich layer may be on the order of a kilometer thick.4 One of the major scientific results of the Phoenix mission was the in situ confirmation of this high-latitude, ground ice reservoir -- sitting literally just beneath the spacecraft (Figure 2.1)
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From page 24... ...
6 It has been suggested that sublimation of some of this water ice during the summertime significantly increases the global abundance of water vapor in the atmosphere.7,8 In addition, investigations from nearly all of the active Mars missions during the past decade have provided complementary evidence for the presence of minerals deposited under past aqueous conditions, some of which still contain chemically bound water. For example, observations with Mars Global Surveyor's Thermal Emission Spectrometer led to the discovery of coarse-grained, crystalline hematite (typically formed in water)
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From page 25... ...
This evidence includes the discovery of: • Small, morphologically "fresh" gullies along the inner walls of many equatorial and midlatitude impact craters;22,23,24 • Valley network and alluvial fan-like features that suggest past rainfall and surface runoff; 25,26 • What appear to be river deltas within closed sedimentary impact basins that might once have held crater lakes;27,28 • Widespread evidence (seen in high-spatial-resolution images) for rhythmically layered sedimentary rocks across much of the planet;29 and • Fine laminations and shallow trough cross-bedding interpreted to have formed by aqueous sedimentation in sulfate-rich outcrops imaged by the Opportunity rover at Meridiani Planum.30,31 MARTIAN METHANE Another significant set of results from Mars that postdates the release of the NRC's 1997 report Mars Sample Return: Issues and Recommendations 32 concerns the spectroscopic detection of methane in the planet's atmosphere both by ground-based telescopes33,34 and by the Mars Express spacecraft.35 Although the spacecraft results are still somewhat controversial, the most recent and definitive ground-based measurements point to the presence of methane in the planet's atmosphere at mixing ratios that vary between <3 parts per billion (by volume)
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From page 26... ...
46 While water vapor was detected in Mars's atmosphere via telescopic measurements and water-ice was unambiguously detected at the martian north pole by Viking,47,48 the first direct measurement of the isotopic composition of water in a martian sample was obtained by the stepwise thermal decomposition and release of water from the SNC meteorites Nakhla and Chassigny.49 That study revealed several important features, including the following: • Water on Mars is not in equilibrium with the host rock, presumably due to the absence of plate-tectonic recycling of the crust; • The composition of water in the martian regolith has evolved over time through groundwater circulation and precipitation of secondary mineral phases; and • The carbonates observed in SNC meteorites were formed on Mars and precipitated by circulating fluids that constitute the subsurface water reservoir. Other measurements of the isotopic systems for hydrogen and carbon have added information about the precipitation of secondary alteration minerals, further refining the understanding of fluid compositions and their evolution.50,51,52,53 The carbon dioxide in the martian atmosphere possesses a highly specific isotopic signature, owing to its interaction with electronically excited atomic oxygen (O 1D)
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From page 27... ...
For example, using this approach, Valley and colleagues observed the same isotopically anomalous oxygen isotopic signature in martian SNC carbonates observed previously by conventional bulk analysis methods. 56,57 These discoveries have provided additional support for the argument that the SNC carbonates record exchanges between the surface regolith and water reservoirs, through subsurface groundwater transport.
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From page 28... ...
In summary, the application of new high-resolution isotopic methods to the study of martian meteorites suggests the following: • Liquid water has existed in the martian subsurface over prolonged periods of geological time; • Active exchanges between surface and subsurface water reservoirs maintained by groundwater circulation provided a means for the active transport of oxidants needed to maintain subsurface redox gradients; and • The abundance of water was sufficient for authigenic mineral precipitation, but relative to the host rock, water volumes have remained low. CONCLUSIONS AND RECOMMENDATIONS The assessment of martian habitability made in the NRC's 1997 report Mars Sample Return: Issues and Recommendations led to its recommendation that: "Samples returned from Mars by spacecraft should be contained and treated as though potentially hazardous until proven otherwise.
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From page 29... ...
Farmer, "The Seasonal and Global Behavior of Water Vapor in the Mars Atmosphere: Com . plete Global Results of the Viking Atmospheric Water Detector Experiment," Journal of Geophysical Research 87:2999-3019, 1982.
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From page 30... ...
. Forget, and the OMEGA Team, "Global Mineralogical and Aqueous Mars History Derived from OMEGA/Mars Express Data," Science 312:400-404, 2006.
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From page 31... ...
Special Regions Science Analysis Group, "Findings of the . Mars Special Regions Science Analysis Group," unpublished white paper, posted June 2006 by MEPAG at http://mepag.jpl.
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